Varactor diodes, typically but not always PN junction devices, are devices whose capacitance varies as a function of the applied reverse voltage. The change in varactor capacitance with applied voltage comes about because the depletion region of the reverse biased junction widens and narrows as the applied voltage is changed. This change in capacitance with applied voltage can be used to advantage to provide tuning and other functions where a voltage variable capacitance is useful. Varactor diodes are much used for this purpose.
However the usefulness of varactors depends upon a number of factors such as the breakdown voltage, the tuning ratio and the series resistance. If a sufficiently large reverse voltage is applied, the varactor will breakdown or fail in generally the same manner as any diode. Obviously, the maximum tuning voltage that can be applied is limited by the breakdown voltage. Hence, the breakdown voltage needs to be higher than the operating voltage. The tuning ratio (TR=Cjv=0/Cjv=V) is defined as the ratio of the capacitance Cjv at one junction voltage, e.g., jv=0 volts, to the capacitance at another junction voltage, e.g., jv =V volts, and is a measure of the amount of capacitance variation that can be usefully obtained. For example, the higher the tuning ratio, the greater the tuning range of an oscillator where the varactor diode capacitance Cjv is controlling the oscillator frequency. So, it is desirable that the tuning ratio (TR) be large. Further, even though the varactor is DC reverse biased and little or no significant DC current flows, it is still carrying AC current so the series resistance contributes to AC losses. Hence, the series resistance of the varactor diode is important since it affects the quality factor (Q) of the varactor as a tuning element. In general, the higher the series resistance, the lower the Q.
It is usually the case with present day varactors that changing the device design or construction to improve one or more of the breakdown voltage, tuning ratio, or series resistance can adversely affect the others. Thus, a need continues to exist for improved varactor structures and methods wherein one or more of these factors can be improved simultaneously without significant adverse effect on other varactor properties.